The invention is directed to an ionization smoke alarm and a method of operating it to distinguish between smoke and contamination.
It is known to detect the increasing aerosol content (smoke) in the air by means of an open ionization chamber. A radioactive member generates an ion current in the ionization chamber which current is decreased by the so-called small ion agglomeration effect if smoke aerosols are present. Conventional ionization smoke alarms give an alarm through the alarm line if a predetermined threshold for the ion current or a potential generated thereby (at the measuring electrode) is exceeded or is not reached. In recent times, more and more so-called analog alarms have been used (German Publication Letter 22 57 931, German Disclosure Letter 29 46 507, EP 0 070 449). According to these alarms, a corresponding signal for the evaluation means used therein is generated in response to the analog value of the measuring chamber current.
Normally, a fire alarm unit consists of a plurality of fire alarms which are connected in groups with a fire alarm central office by means of current supply lines and signal lines. The evaluation of the analog signals makes necessary an associated definite identification signal for each alarm as well as for its respective measuring value. An output of analog signals in short time intervals is necessary in order to recognize a fire immediately. Since a great number of fire alarms are normally connected to a common cable, an agglomeration of signals result. Both a high-grade alarm identification word consisting of a signal sequence and an identification data word containing the associated analog value for each alarm, as well as a high-grade cable network, are all absolutely necessary for a safe communication to the central office which is often far away from the alarms (EP 0 121 048 or also EP 0 070 449). Also in the central office relatively high expenses are necessary for the data processing of the plurality of signal sequences (EP 0 067 339).
These expenses are made in order to recognize as early as possible modifications of the measuring chamber current which are not caused by fire and to avoid false alarms (German Publication Letter 22 57 931 or German Disclosure Letter 29 46 507).
Apart from climatic influences, as for instance temperature, pressure etc., as well as aging effects, especially of the radioactive member, the correct operation of such smoke alarms is influenced by contamination which naturally varies considerably depending on which atmosphere the alarm is exposed to. One distinguishes substantially between two detrimental effects which are based upon different contamination results. If the contamination at the insulation of the structure supporting the measuring electrode predominates, a reduction of the responsiveness or even a non-response results on account of leakage currents. In order to detect this condition in time, solutions have been already proposed (German Patent Letter 20 29 794, EP 0 033 888, German Disclosure Letter 30 04 753 or German Patent Letter 20 04 584).
However, if a contamination of the radioactive member predominates, for instance on account of dirt depositions, a reduction of the measuring chamber current results on account of a reduction of the movement energy or of the ionization capability of the radioactive radiation; the ionization smoke alarm becomes more sensitive with respect to smoke. If the continuing contamination of the radioactive member is not recognized, a false alarm results if corresponding precautions are not taken.
Several solutions have been already suggested for detecting this highly critical condition of an alarm very early. So, for instance, with conventionally operating threshold alarms, one or a plurality of additional pre-alarm thresholds are provided which give an alarm for relatively small chamber current decreases (Swiss Patent Letter 629 905 or Swiss Patent Letter 574 532). In order to check the function of the ionization smoke alarms from the central office, or to determine the actual responsiveness or, more precisely stated, to determine the voltage difference which has to be overcome for giving an alarm at the measuring electrode, it has been already proposed to either continuously, or by steps, increase the voltage at the outer electrode of the measuring chamber (German Publication Letter 20 19 791, German Patent Letter 202 764 or German Patent Letter 20 50 719). Furthermore, it already has been suggested in German Disclosure Letter 21 21 382 to evaluate only such modifications of the measuring chamber current which extend for longer periods to determine whether smoke or, alternatively, dirt is the reason for a chamber current variation. Here, very slow variations of the current are attributed to the influence of dirt. Furthermore, in the last-cited publication, the installation of a radiation detector is also proposed with which the radioactivity is directly measured in order to be able to immediately identify variations of the ionization capacity. In the same publication the installation of assisting electrodes is described either to be able to better recognize or compensate an increase of the insulation leaking current.
It is also known from EP 0 121 048 to provide each ionization smoke alarm with so-called noise levels. Here, additional thresholds are formed below the alarm threshold, and additionally a superimposed long-time drifting is taken into account. A comparable method as also become known for analog alarms (EP 0 070 449).
Furthermore, it has become known from EP 0 067 339 to use modifications of the static current of the measuring chamber caused by varying environmental conditions as criterion whether the alarm is in a correct operational condition.
However, all the known methods do not shown any way to determine, in a sufficiently safe manner, whether dirt depositions on the radioactive member or floating smoke aerosols are the reason for a reduction of the measuring chamber current. The reaction of an alarm at so-called pre-alarm thresholds makes necessary an examination as to whether a fire has developed which has to be carried out directly by a person, i.e. an extensive alarm organization is required for a responsible user. Indeed, in most of the cases, contamination is the reason for triggering of the alarm based on the pre-alarm threshold; however, the danger is present that attentiveness is reduced thereby or that at least a great loss of confidence is caused. Fire alarms which does not provide an alarm for a relatively slow variation of the measuring chamber current bring the danger that they detect slowly smoldering fires very late or that they do not detect them at all. A short-time contamination of a considerably amount such as for instance, a dewing of the radioactive radiators, cannot be distinguished by means of this method from a current variation in the measuring chamber caused by a fast smoke increase.
On principle, the known analog systems also have these above-cited deficiencies. Also, even with considerable high technical efforts, only a few of the actually existing defects which are simulated by contamination can be detected. With most of the known solutions concerning analog alarms, either contamination or aging is imputed if the measuring chamber current values change very slowly, or if a rather worthless evaluation of the variations of the measuring chamber current occurring during normal operation is carried out.
It is the object of the present invention to provide an method of operating a ionization smoke alarm with which it can be recognized in a safe manner whether a modification of the measuring chamber current is caused by the entering of smoke aerosols or instead by contamination or other deteriorations of the radioactive source.